What Is Semantic / Structural Markup and Why Does It Matter?

I always found the definition of “Semantic / Structural Markup” murky on the intertubes. I thought Jeffrey Zeldman described it well in his book Designing with Web Standards.

What Is Semantic / Structural Markup?

Markup is “semantic” when tags are chosen according to what they mean. For example, tagging a headline h1 because it is the most important headline on the page is a semantic authoring practice. Tagging a headline h1 “to make it look big” is not. … I use the phrase “structural markup” to mean pretty much the same thing as “semantic markup.” (“Structural markup” takes its name specifically from the idea that the web document has an outline-like structure.) – Jeffrey Zeldman, Designing with Web Standards

Zeldman goes on to make many great points on why semantic markup matters, here’s my paraphrase.